Mar Cuadrado-Tejedor

Phenylbutyrate ameliorates cognitive deficit and reduces tau pathology in an Alzheimer's disease mouse model.

Chromatin modification through histone acetylation is a molecular pathway involved in the regulation of transcription underlying memory storage. Sodium 4-phenylbutyrate (4-PBA) is a well-known histone deacetylase inhibitor, which increases gene transcription of a number of genes, and also exerts neuroprotective effects. In this study, we report that administration of 4-PBA reversed spatial learning and memory deficits in an established mouse model of Alzheimers disease (AD) without altering β-amyloid burden. We also observed that the phosphorylated form of tau was decreased in the AD mouse brain after 4-PBA treatment, an effect probably due to an increase in the inactive form of the glycogen synthase kinase 3β (GSK3β). Interestingly, we found a dramatic decrease in brain histone acetylation in the transgenic mice that may reflect an indirect transcriptional repression underlying memory impairment. The administration of 4-PBA restored brain histone acetylation levels and, as a most likely consequence, activated the transcription of synaptic plasticity markers such as the GluR1 subunit of the AMPA receptor, PSD95, and microtubule-associated protein-2. The results suggest that 4-PBA, a drug already approved for clinical use, may provide a novel approach for the treatment of AD.

Phenylbutyrate rescues dendritic spine loss associated with memory deficits in a mouse model of Alzheimer disease

Alzheimers disease (AD) and ageing are associated with impaired learning and memory, and recent findings point toward modulating chromatin remodeling through histone acetylation as a promising therapeutic strategy. Here we report that systemic administration of the HDAC inhibitor 4‐phenylbutyrate (PBA) reinstated fear learning in the Tg2576 mouse model of AD. Tg2576 mice develop age‐dependent amyloid pathology and cognitive decline that closely mimics disease progression in humans. Memory reinstatement by PBA was observed independently of the disease stage: both in 6‐month‐old Tg2576 mice, at the onset of the first symptoms, but also in aged, 12‐ to 16‐month‐old mice, when amyloid plaque deposition and major synaptic loss has occurred. Reversal of learning deficits was associated to a PBA‐induced clearance of intraneuronal Aβ accumulation, which was accompanied by mitigation of endoplasmic reticulum (ER) stress, and to restoration of dendritic spine densities of hippocampal CA1 pyramidal neurons to control levels. Furthermore, the expression of plasticity‐related proteins such as the NMDA receptor subunit NR2B and the synaptic scaffold SAP102 was significantly increased by PBA. Our data suggest that the beneficial effects of PBA in memory are mediated both via its chemical chaperone‐like activity and via the transcriptional activation of a cluster of proteins required for the induction of synaptic plasticity and structural remodeling.

Rosiglitazone rescues memory impairment in Alzheimer's transgenic mice: mechanisms involving a reduced amyloid and tau pathology.

Clinical studies suggest that agonists at peroxisome proliferator-activated receptor gamma (PPARγ) may exert beneficial effects in patients with mild-to-moderate Alzheimers disease (AD), but the mechanism for the potential therapeutic interest of this class of drugs has not yet been elucidated. Here, in mice overexpressing mutant human amyloid precursor protein, we found that chronic treatment with rosiglitazone, a high-affinity agonist at PPARγ, facilitated β-amyloid peptide (Aβ) clearance. Rosiglitazone not only reduced Aβ burden in the brain but, importantly, almost completely removed the abundant amyloid plaques observed in the hippocampus and entorhinal cortex of 13-month-old transgenic mice. In the hippocampus, neuropil threads containing phosphorylated tau, probably corresponding to dystrophic neurites, were also decreased by the drug. Rosiglitazone switched on the activated microglial phenotype, promoting its phagocytic ability, reducing the expression of proinflammatory markers and inducing factors for alternative differentiation. The decreased amyloid pathology may account for the reduction of p-tau-containing neuropil threads and for the rescue of impaired recognition and spatial memory in the transgenic mice. This study provides further insights into the mechanisms for the beneficial effect of rosiglitazone in AD patients.

Phosphodiesterases as Therapeutic Targets for Alzheimer's Disease

Alzheimers disease (AD) is the most common form of dementia among the elderly. In AD patients, memory loss is accompanied by the formation of beta-amyloid plaques and the appearance of tau in a pathological form. Given the lack of effective treatments for AD, the development of new management strategies for these patients is critical. The continued failure to find effective therapies using molecules aimed at addressing the anti-beta amyloid pathology has led researchers to focus on other non-amyloid-based approaches to restore memory function. Promising non-amyloid related candidate targets include phosphosdiesterases (PDEs), and indeed, Rolipram, a specific PDE4 inhibitor, was the first compound found to effectively restore cognitive deficits in animal models of AD. More recently, PDE5 inhibitors have also been shown to effectively restore memory function. Accordingly, inhibitors of other members of the PDE family may also improve memory performance in AD and non-AD animal models. Hence, in this review, we will summarize the data supporting the use of PDE inhibitors as cognitive enhancers and we will discuss the possible mechanisms of action underlying these effects. We shall also adopt a medicinal chemistry perspective that leads us to propose the most promising PDE candidates on the basis of inhibitor selectivity, brain distribution, and mechanism of action.

Sildenafil restores cognitive function without affecting β-amyloid burden in a mouse model of Alzheimer's disease.

BACKGROUND AND PURPOSE Inhibitors of phosphodiesterase 5 (PDE5) affect signalling pathways by elevating cGMP, which is a second messenger involved in processes of neuroplasticity. In the present study, the effects of the PDE5 inhibitor, sildenafil, on the pathological features of Alzheimers disease and on memory‐related behaviour were investigated.

Early Changes in Hippocampal Eph Receptors Precede the Onset of Memory Decline in Mouse Models of Alzheimer's Disease

Synapse loss occurs early in Alzheimers disease (AD) and is considered the best pathological correlate of cognitive decline. Ephrins and Eph receptors are involved in regulation of excitatory neurotransmission and play a role in cytoskeleton remodeling. We asked whether alterations in Eph receptors could underlie cognitive impairment in an AD mouse model overexpressing human amyloid-beta protein precursor (hA beta PP) with familial mutations (hA beta PP swe-ind mice). We found that EphA4 and EphB2 receptors were reduced in the hippocampus before the development of impaired object recognition and spatial memory. Similar results were obtained in another line of transgenic A beta PP mice, Tg2576. A reduction in Eph receptor levels was also found in postmortem hippocampal tissue from patients with incipient AD. At the time of onset of memory decline inhA beta PP swe-ind mice, no change in surface expression of AMPA or NMDA receptor subunits was apparent, but we found changes in Eph-receptor downstream signaling, in particular a decrease in membrane-associated phosho-cofilin levels that may cause cytoskeletal changes and disrupted synaptic activity. Consistent with this finding, Eph receptor activation in cell culture increased phosho-cofilin levels. The results suggest that alterations in Eph receptors may play a role in synaptic dysfunction in the hippocampus leading to cognitive impairment in a model of AD.

Overexpression of wild-type human APP in mice causes cognitive deficits and pathological features unrelated to Aβ levels

Transgenic mice expressing mutant human amyloid precursor protein (APP) develop an age-dependent amyloid pathology and memory deficits, but no overt neuronal loss. Here, in mice overexpressing wild-type human APP (hAPP(wt)) we found an early memory impairment, particularly in the water maze and to a lesser extent in the object recognition task, but beta-amyloid peptide (Abeta(42)) was barely detectable in the hippocampus. In these mice, hAPP processing was basically non-amyloidogenic, with high levels of APP carboxy-terminal fragments, C83 and APP intracellular domain. A tau pathology with an early increase in the levels of phosphorylated tau in the hippocampus, a likely consequence of enhanced ERK1/2 activation, was also observed. Furthermore, these mice presented a loss of synapse-associated proteins: PSD95, AMPA and NMDA receptor subunits and phosphorylated CaMKII. Importantly, signs of neurodegeneration were found in the hippocampal CA1 subfield and in the entorhinal cortex that were associated to a marked loss of MAP2 immunoreactivity. Conversely, in mice expressing mutant hAPP, high levels of Abeta(42) were found in the hippocampus, but no signs of neurodegeneration were apparent. The results support the notion of Abeta-independent pathogenic pathways in Alzheimers disease.

Sildenafil protects against 3-nitropropionic acid neurotoxicity through the modulation of calpain, CREB, and BDNF.

In this study we tested whether phosphodiesterase 5 (PDE5) inhibitors, sildenafil and vardenafil, would afford protection against 3-nitropropionic acid (3NP), which produces striatal lesions that closely mimic some of the neuropathological features of Huntingtons Disease (HD). The neurotoxin was given over 5 days by constant systemic infusion using osmotic minipumps. Animals treated with PDE5 inhibitors (sildenafil or vardenafil) showed improved neurologic scores, reduced the loss of striatal DARPP-32 protein levels and lesion volumes, and decreased calpain activation produced by 3NP. This protective effect was independent of changes in 3NP-induced succinate dehydrogenase inhibition. Furthermore, striatal p-CREB levels along with the expression of BDNF were significantly increased in sildenafil-treated rats. In summary, PDE5 inhibitors protected against 3NP-induced striatal degeneration by reducing calpain activation and by promoting survival pathways. These data encourage further evaluation of PDE5 inhibitors in transgenic mouse models of HD.

Enhanced Expression of the Voltage-Dependent Anion Channel 1 (VDAC1) in Alzheimer's Disease Transgenic Mice: An Insight into the Pathogenic Effects of Amyloid-β

The mitochondrial voltage-dependent anion channel 1 (VDAC1) is involved in the release of apoptotic proteins with possible relevance in Alzheimers disease (AD) neuropathology. Through proteomic analysis followed by Western blotting and immunohistochemical techniques, we have found that VDAC1 is overexpressed in the hippocampus from amyloidogenic AD transgenic mice models. VDAC1 was also overexpressed in postmortem brain tissue from AD patients at an advanced stage of the disease. Interestingly, amyloid-β (Aβ) soluble oligomers were able to induce upregulation of VDAC1 in a human neuroblastoma cell line, further supporting a correlation between Aβ levels and VDAC1 expression. In hippocampal extracts from transgenic mice, a significant increase was observed in the levels of VDAC1 phosphorylated at an epitope that is susceptible to phosphorylation by glycogen synthase kinase-3β, whose activity was also increased. The levels of hexokinase I (HXKI), which interacts with VDAC1 and affects its function, were decreased in mitochondrial samples from AD models. Since phospho-VDAC and reduced HXKI levels favors a VDAC1 conformational state more prone to the release proapoptotic factors, regulation of the function of this channel may be a promising therapeutic approach to combat AD.

Insulin-like growth factor 2 reverses memory and synaptic deficits in APP transgenic mice.

Insulin‐like growth factor 2 (IGF2) was recently found to play a critical role in memory consolidation in rats and mice, and hippocampal or systemic administration of recombinant IGF2 enhances memory. Here, using a gene therapy‐based approach with adeno‐associated virus (AAV), we show that IGF2 overexpression in the hippocampus of aged wild‐type mice enhances memory and promotes dendritic spine formation. Furthermore, we report that IGF2 expression decreases in the hippocampus of patients with Alzheimers disease, and this leads us to hypothesize that increased IGF2 levels may be beneficial for treating the disease. Thus, we used the AAV system to deliver IGF2 or IGF1 into the hippocampus of the APP mouse model Tg2576 and demonstrate that IGF2 and insulin‐like growth factor 1 (IGF1) rescue behavioural deficits, promote dendritic spine formation and restore normal hippocampal excitatory synaptic transmission. The brains of Tg2576 mice that overexpress IGF2 but not IGF1 also show a significant reduction in amyloid levels. This reduction probably occurs through an interaction with the IGF2 receptor (IGF2R). Hence, IGF2 and, to a lesser extent, IGF1 may be effective treatments for Alzheimers disease.